1. Field of the Art
This invention relates to an interferometric surface inspection machine for inspecting surface conditions of finished spherical lens elements (hereinafter referred to as "lens specimens") by the use of a laser interferometer.
2. Description of the Prior Art
It has been the general practice in the art to detect the surface irregularities of a finished lens by the use of a test plate, more specifically, by reflecting light on the surface of a lens specimen through a test plate and measuring the number of lines in an interference fringe pattern (normally referred to as the number of Newton's fringes which is formed as a result of interference between reflected light from a reference surface of the test plate with reflected light from the lens surface under testing. In this regard, the simplest way of inspection is to overlay a test plate directly on a lens surface to be inspected, and project light on the test plate to observe with naked eyes the interference fringe which is formed by the two light reflections from the reference surface of the test plate and the lens surface under inspection. However, in such lens surface inspection, the test plate which is placed in direct contact with an inspecting lens surface has to be handled very carefully to avoid damages to both of the contacting surfaces. Especially, on the part of the test plate which is used repeatedly, it is very likely that accumulation of scratches take place on the entire surface of the test plate even from an extremely early time point of the inspection, necessitating replacement of the test plate at a high frequency. In the surface inspection of this sort, there has been another problem that, unless the surfaces of the test plate and lens specimen are cleaned prior to each inspection, accurate observation of the interference pattern may be jeopardized by dust or foreign matter which has crept onto their overlapped surface portions.
In this connection, there has been known, for example, from U.S. Pat. No. 5,127,734, a surface inspection apparatus which permits to inspect a lens surface in a non-contacting state by the use of a laser interferometer. More specifically, the surface inspection apparatus employs a laser generator to project a laser beam on a reference lens through an optical laser beam guide system, reflecting part of the laser beam light on a reference surface of the reference lens and reflecting part of the laser beam, which has passed through the reference lens, on a surface of a lens specimen which is located in face to face relation with the reference lens, and observing the interference fringe which is produced by interference between the two light reflections.
In this manner, in case of a laser interferometer, the surface of a lens specimen is held in non-contacting state with the reference lens during inspection, preventing the above-mentioned problems of the contacting inspection by the use of a test plate in which the overlapped surfaces of the test plate and an inspecting lens specimen, in direct contact with each other, are both vulnerable to damages or flaws. In the above-described prior art which is arranged to irradiate an inspecting lens surface with light projected from above, however, the lens specimen under inspection needs to be supported in position for inspection on the side away from the lens surface under inspection or needs to be clamped in position at its peripheral edge portions. Normally, the lens specimens to be inspected are manufactured with a certain degree of tolerances in thickness and outer diameter, so that it has been imperative for an accurate inspection to make adjustment of the relative positions of the interferometer and the inspecting lens surface each time a new lens specimen is set on a lens mount member for inspection. For this adjustment of relative positions, usually the interferometer proper is moved in the direction of the optical axis, while the lens mount member which supports a lens specimen is moved in a direction perpendicular to the optical axis and at the same time turned relative to the optical axis to correct its inclination if any. Such adjustment of relative positions of the interferometer and the inspecting lens surface has to be carried out for each of the individual lens specimens even if they are of the same kind. The job of pre-adjusting the position of each lens specimen relative to the interferometer is very troublesome and requires meticulous skills, making it extremely difficult to realize automatic surface inspecting operations.